1. Field of the Invention
This invention relates to a power converter using an NPC inverter (Neutral Point Clamped Inverter), and more particularly to a power converter which can suppress the fluctuation of a neutral potential of a three-level DC voltage source.
2. Description of the Related Art
One example of a main circuit of a conventional power converter using an NPC (Neutral Point Clamped Type) inverter is shown in FIG. 15. In FIG. 15, AC voltage supplied from a three-phase AC power source 41 is converted to DC voltage by a converter 1, divided by capacitors 4, 5. Here, capacitors 4, 5 forms a three-level DC voltage source and DC voltages thereof having a positive side potential VP, a neutral potential VO and a negative side potential VN are output. DC voltages having this neutral potential VO are converted to a three-phase AC voltage of prescribed frequency by an NPC inverter 2 for driving an AC motor 3. NPC inverter 2 is known as an inverter to convert DC voltages having neutral potential VO to AC power with less higher harmonics.
In this case, AC power supplied to AC motor 3 from a positive side voltage (VP-VO) and a negative side voltage (VO-VN) by NPC inverter 2 is not equal at any instantaneous time and fluctuates at frequency that is 3 times of an output frequency of NPC inverter 2. Converter 1 is able to control only positive-negative voltage (VP-VN) and therefore, neutral potential VO also fluctuates at 3 times of the output frequency. When neutral voltage VO fluctuates, the feature of the NPC inverter to suppress higher harmonics is lost, and therefore, various methods are being studied to suppress the fluctuation of neutral potential VO.
For instance, such a control system is used to suppress fluctuation of neutral potential VO as "PWM System of Three-Level GTO Inverter", Industrial Application Section No. 85 disclosed at the National Meeting of the Institute of Electrical Engineers of Japan, 1994, as follows. That is, voltages in three phases are biased while line voltages of three-phase NPC inverter kept unchanged, and powers supplied from positive side voltage (VP-VO) and negative side voltage (VO-VN) are balanced by shifting this bias voltage to positive and negative sides in a short cycle.
Specifically, to cope with this fluctuation of neutral potential of a three-level DC voltage source, a method to apply bias to output voltage reference of an NPC inverter was so far used. FoP instance, if positive side voltage of a DC voltage source becomes larger than negative side voltage, positive bias is applied to voltage reference. As a result, positive side DC power consumption increases more than negative side DC power consumption and thus, positive side and negative side DC voltages can be balanced.
A definite construction will be explained referring to FIG. 15. From the difference between a positive side voltage Vd1 and a negative side voltage Vd2 obtained from a positive side DC voltage detector 20 to detect the voltage of positive side capacitor 4 and a negative side DC voltage detector 21 to detect the voltage of negative side capacitor 5, a positive-negative differential voltage is obtained and is input to a bias regulator 44. Fluctuation of neutral potential was suppressed by controlling NPC inverter 2 based on the sums of this positive-negative differential voltage and three-phase voltage references VU*, VV* and VW* computed by a three-phase voltage reference computing unit 40.
However, according to this conventional method, as bias is applied to output voltage of NPC inverter 2, when heavy loaded (when overcurrent is applied), voltage may restricted and bias may not be compensated in some case. In this case, there is such a problem that compensation of neutral potential fluctuation is not preferentially controlled but effectively controlled only when sufficient output voltage is available, and if large load current flows in a moment because of sudden change in load, etc., neutral potential fluctuates largely and overvoltage/overcurrent is induced.
Furthermore, according to such a system to suppress neutral potential fluctuation by unbalancing positive and negative side voltage consumptions by applying a bias to three-phase voltage references, voltage actually given as three-phase line voltage will become smaller than DC link voltage by bias. Therefore, voltage utilization factor drops and an NPC inverter with large voltage capacity becomes necessary. As a result, a power converter system as a whole will become large.